For a variety of reasons, modem communication equipment is now being designed to process data at progressively higher speeds. Perhaps the most significant reason relates to the need to transfer video information between computers. Such data transfer has grown exponentially in recent years, and no decrease in growth is anticipated in the foreseeable future. In order to handle this growth, only optical circuitry appears capable of meeting the demand because of the enormous bandwidth that an optical fiber can provide. Nevertheless, distribution equipment is still needed to route optical signals to the same pre-existing locations, and this means that optical connecting hardware needs to be sufficiently small to accommodate large numbers of individual fiber connections. Thus, while high speed and large bandwidth are achievable, the amount of space, or “real estate” available for making necessary connections of the fiber is limited, thereby limiting the distribution potential of the system.
One particular location where congestion occurs is on circuit boards that contain optical components (i.e., optical circuit boards or PCBs) where individual input/output ports must be provided to make connections on a per-fiber basis. Moreover, it is desirable to plug these circuit boards into a panel, or backplane, that accommodates a number of other circuit boards similar to the way electrical circuit boards are mounted in an equipment bay each in its designated slot. However, electrical connections can be easily configured to accommodate the available space, while the primary vehicle for optical connection is the “butt” connector where the end face of one fiber is butted against the end face of another fiber. In such a connection, there should be no air gap between the fiber end faces and there should be no fiber displacement in the X and Y directions—otherwise there would be too much signal loss. (It is noted that a singlemode optical fiber has a light-carrying region that is only about 8 microns (μm) in diameter, and that it must be precisely aligned in an axial direction with another fiber.) It is therefore a challenging task to provide a number of optical devices on a plug-in optical circuit board that accurately mate with a corresponding number of stationary optical connectors.
Optical devices are known that might be adapted to mount on an optical circuit board, but their construction is relatively complex and/or their attachment to an optical circuit board requires expensive and time-consuming manual labor. More importantly, there is a need to standardize the optical interface for plug-in optical circuit boards. The interface should provide accurate optical alignment and be suitable for high density interconnection.
Inasmuch as the fiber density in a backplane has increased dramatically with the development of all optical networking in transmission systems, prior art backplane connectors pose a limitation on the number of fiber paths per printed circuit board and slot arrangements in a shelf or cabinet. In many instances, previous transmission systems have generally employed two fiber optic paths per card (circuit board). Efforts to increase the number of paths have led to arrangements having eight fiber paths per card/slot, but it is expected that future systems may have as many as thirty-four or even more fiber paths. The complexity of PCB (printed circuit board) card increases in direct proportion to the number of additional fiber paths, which severely limits the available real estate for accommodating the individual fiber optic connector components. In U.S. Pat. No. 6,402,393B1 of Grimes, et al., there is shown a system wherein several duplex connectors are arrayed along a side edge of a circuit board and are positioned to mate with corresponding receptacles or adapters mounted on the backplane. Inasmuch as positioning of each connector and its corresponding adapter must be precise for optimum signal transfer, such an arrangement necessarily occupies a large amount of available real estate on the card or circuit board while limiting the number of optical paths that can be accommodated.
Plug-in circuit boards that house optical components will soon be as familiar as circuit boards that house electrical components. The plug-in concept has been widely accepted because such circuit boards provide a large and manageable amount of hardware on an easily replaceable device. Circuit boards frequently include diagnostic hardware and software that can alert service personnel when a board is not working properly. And because a defective board can be quickly replaced by pulling out one circuitry board and plugging in another, maintenance is facilitated and downtime is minimal. This is particularly useful in large and complex systems where removal of a single board affects a large number of customers. The convenience of plug-in circuit boards is largely attributable to the fact that all connections between the board and a backplane can be non-destructively severed by merely pulling the circuit board from the slot where it operatively resides. By “backplane” is meant, generally, a wall that separates internal apparatus from external apparatus, and through which a connection(s) is made. It necessarily follows, therefore, that connections of a multiplicity of signal paths must be anticipated and the necessary hardware developed, which does not require exorbitant real estate on the PCB, for example.